Sheet metal forming embraces a wide array of manufacturing processesTypically work with thin sheets; usually steel

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A sheet metal part produced in presses is called a stamping [(a) above]

Stamp ‘to force downward” “to pound”

Low carbon steel is the most common sheet metal material because of its low cost and generally good strength and formability characteristics

Aluminum is the most common s/m material where corrosion resistance is a concern; beverage cans, kitchen utensils

Most s/m processes are performed at room temperature

Stamped parts generally formed in one compression operation

Spun parts (“spinning”) generally formed individually, by shaping a blank over a rotating mandrel; expensive unless performed automatically

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Summary of characteristics of many of the sheet metal forming processes

These processes considered throughout the chapter

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One process involves punching

This is actually a shearing operationSometimes the product is the part that is punched out and sometimes it is the part remaining

Note that the profile of the punched edge is not uniform, (b)

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Clearance is a major factor in determining shape and quality of sheared edgeAs clearance increases, the zone of deformation becomes larger, and the sheared edge becomes rougher-Removal of resultant burrs incurs more work and more costIncrease speed to increase edge qualityWith increasing speed, the heat generated by plastic deformation is confined to a smaller and smaller zonesmaller and smaller zone

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Punching (piercing) : the sheared slug is scrapBlanking: keep (use) the slug & scrap the restThe green is kept (in above illustrations)

Die cutting: A shearing operation that consists of elements in (c):Perforating: Punching a number of holes in a sheetParting: Shearing the sheet into two or more piecesNotching: Removing pieces or various shapes from the edgesLancing: Leaving a tab without removing any material

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Fine Blanking

(a): Compare traditional and fine blanking

Can produce very smooth and square edges by fine blankingNote V shaped stinger to hold part in position

Fine blanking employs clearances approx. 1% of the thickness of the sheet- Compare normal clearances 2% to 10%

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SlittingA shearing operation performed by a pair of circular bladesBlades may follow straight or curved lineStill have a burrCan result in distortions of the sheared edges

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An example of Taylor-welded blanks (keep the green)First:: Five different pieces created by blanking

These pieces may be different materials and thicknessesSecond: Five pieces joined by laser butt (edge to edge) weldingThird: Stamp to final shape

It is important that the pieces be properly aligned prior to welding because of the thin sections involved

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Examples of parts for automobiles using the Taylor-welded technology

Note different thicknesses in laser welded components

Note capability to produce a variety of relatively thin but structurally strong shapes

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Shaving: An operation to remove burrs and create smoother sheared edges

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If surfaces of both surfaces of the punch and die are both flat then may require large shear force to simultaneously cut all the edges.

Several strategies to reduce this force:Beveling either punch or die face (note (c) and (d))Desirable to have symmetry to have equal distribution of the forces

(b) Is an example of non symmetrical and this causes lateral forces which must be contained

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Compound DiesUsed to perform several operations on the same sheet in one operationThis example: (a) & (b): Outside perimeter of washer is cut first; followed by cutting the inside diameter, all in one stroke

(c) Illustrates a different variation: the washer is cut in two stages, i.e. each washer requires two strokesrequires two strokes

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Stress-strain diagrams show different patterns for various metals & alloysSome metals have a zone of yield point elongationThis occurs in a zone within which the metal is stretched unevenly between upper and lower yield points and exhibits stretch marks. This behaviour results in Luder’s bands.

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Cupping TestThe earliest tests developed to predict s/m formabilityThe round punch or steel ball is pressed onto specimen until crack forms; then

measure punch depth, d.In (b), note the distortion at edges of narrower test strips(a) Erichsen Test(b) Bulge Test

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Flat specimens are cut to various widths, then tested (previous illustration)The surfaces of the flat specimens are marked in circles, prior to testingAfter testing, the resultant shape may be still circular, but more likely, elliptical.The deformation along major & minor axes are measured. Major axis strain is always positive (larger diameter than original); minor axis strain may be positive or negative,The FLD Forming Limit Diagram is constructed showing the boundaries betweenThe FLD, Forming Limit Diagram, is constructed showing the boundaries between failure (higher) & safe (lower) regions

Example: AluminumA: Major +20%; Minor -10% = to lower left (“below”) curve, therefore there would be no tear in that location of the specimenB: Major +80%; Minor -40% = to above right of curve; therefore there would be tear in that location of the specimen

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Illustrates the deformation discussed on previous slide

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Bending is one of the most common industrial forming operationsBending imparts strength to flat (or curved) sheet.

Edges, ribbed, rolled contours

This diagram shows the terminology in bending

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Look carefully at (c) to see the reduction in bend length at the bend.

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Minimum Bend RadiusThe radius at which a crack first appears at the outer fibres of a sheet being bent.

The bend radius is usually expressed as a reciprocal of the thickness.Example:‘3T’ means the smallest radius to which the sheet can be bent without cracking is three times the thickness of a sheet

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R = T(50/r -1)Where R = bend radius

T = thicknessr = tensile reduction of area of the sheet metal (expressed as a %)

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Compensate for springback by overbending the part

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(a) Overbending(b) Overbending(c) Bottoming the punch by ‘coining’ to produce local compressive forces(d) Bottoming

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Beading

The periphery of the s/m is bent into the cavity of the die

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Flanging

Bending edges of s/m, usually 90o

Shrink FlangeSubject flange to compressive hoop forces

Stretch FlangeTensile forces

Reverse FlangeAlternate shrink & stretch

DimplingThe hole is first punched, then expanded into a flange

PiercingCan create flanges with a shaped punch

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g p p

Can be used to create a wide variety of cross sectional shapes as continuous lengths, later cut to length

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Bending tubes

Often requires special tooling because of the tendency for buckling & folding

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BulgingPlace a tubular, conical, or curvilinear part into a split-female die and then expanding the part, usually with a polyurethane plugPunch is then retracted; plug returns to its original shape, and the formed part is removed by opening the split dies

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Stretch FormingThe s/m is clamped along its edges and then stretched over a male dieThe die moves upward, downward, or sidewaysIn most cases the blank is a rectangular sheet clamped along its narrower edges and stretched lengthwise, thus allowing the material to shrink in width.

Controlling the amount of stretching is important to prevent tearing

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Deep DrawingThe name refers to the capability to produce deep parts, although many shallow depth parts are producedDeep drawing is important due to widespread use.

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Deep DrawingImportant variables are:-Properties of s/m-Ratio of Blank diameter, Do, to Punch diameter, Dp, Do/ Dp,

-Clearance between punch & die, c-Punch radius, Rpp

-Die-corner radius, Rd

-Blankholder force-Friction-Lubrication

-These are all independent variables (except F)

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AnisotropyCold rolling of a sheet results in anisotropy by preferred orientation or by mechanical fibering due to the alignment of any impurities, inclusions, and voids that may be present

For a tensile test, length, width, and thickness strains occur and the strains in the width and thickness directions need not be equal r = strain / straintwidth and thickness directions need not be equal. r strainw / straint

Important in deep drawing because of the high amount of stretching undergone by the metal

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The Normal anisotropy, Ravg, can be calculated for a sheet using R45 and R90g

Normal anisotropyr0 = r45 = r90 ≠ 1

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LDR = Maximum blank diameter = Do

Punch diameter Dp

Note: both scales are logarithmic

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The metal can stretch more in direction of rolling

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Draw beads are often necessary to control the flow of the blank into the die cavityBeads restrict the free flow of the s/m by bending and unbending it during the drawing cycle, thereby increasing the force required to pull the sheet into the die cavityDraw beads are useful in drawing box-shaped and nonsymmetric parts. Various regions undergo different types of deformation during drawing

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Case Study

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In rubber forming, one of the dies (the female die) is made of a flexible material, typically a polyurethane membrane. Polyurethanes are selected because of their abrasion resistance, fatigue life, resistance to cutting & tearing.

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In the hydroform process, the pressure over the rubber membrane is controlled throughout the forming cycle.This procedure allows close control of the part during forming and prevents wrinkling or tearing.Deeper draws are obtained than during regular deep drawing because the pressure around the rubber membrane forces the cup against the punch.This increases the friction which reduces the longitudinal tensile stresses in the cupThis increases the friction which reduces the longitudinal tensile stresses in the cup and thus delays fracture

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In tube hydroforming, metal tubing is formed in a die and pressurized internally by a fluid, usually water

This process can produce intricate hollow shapes

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Case StudyNote varying cross sections – weight reductionMost components made by tube hydroforming, holes perforated, then components are assembled through robotic gas-metal arc welding

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Case Study

Sequence in formation of one component by tube hydroforming

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Case Study

Used in the components in case study

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Case Study

The machine

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Spinning

A process that involves the forming of axisymmetric parts over a mandrel by the use of various tools and rollers

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Shear spinning

This operation produces an axisymmetric conical or curvilinear shape

Prefer two rollers applied oppositely, to balance the forces acting on the mandrel

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Incremental forming

A class of processes that are related to conventional metal spinning

(a) The rotating blank is deformed by a steel rod with a smooth hemispherical tip to produce axisymmetric parts

(b) A part that has been produced by CNC incremental forming.The part does not have to be axisymmetric

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Examples of products made by combining diffusion bonding with superplastic forming. (SPF/DB).

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Explosive Forming

The s/m blank is clamped over a die and the entire assembly is lowered into a tank filled with water. The air in the die cavity is evacuated, an explosive charge is placed at a certain height and the charge is detonated.The explosion generates a shock wave with a pressure that is sufficient to form sheet metalssheet metals

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In Electromagnetically Assisted Forming the energy stored in a capacitor bank is discharged rapidly through a magnetic coil. In a typical example, a ring shaped coil is placed over a tubular workpiece.The tube is then collapsed by magnetic force over a solid piece, thus making the assembly an integral part.The mechanics of the process is based on the fact that a magnetic field produced by the coil crosses the metal tube and generates eddy currents in the tubeby the coil crosses the metal tube and generates eddy currents in the tube. In turn, these eddy currents produce their own magnetic firld.The forces produced by the two magnetic fields oppose each other.The repelling force generated between the coil and the tube then collapses the tube over the inner piece

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Case Study

Cymbals can be made with a variety of sizes, weights, hammerings and surface finishes

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Case Study

First cast, then cooledRolled successively (up to 14 times)Roll the bronze at a different angle with each pass to minimize anisotropyHeat and then stretch formed into the bell shape that determines cymbal’s tonesFollow with centre drilling of hole for hanging and trimmed on a rotary shear to approximate final diameters.Another stretch forming step that determines the characteristic form that controls the pitch.

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Case Study

Peening & hammering

Work hardening Stress reliefToneAppearance

Automatic or manual

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Manufacturing metal honeycomb structuresCommon in nonmetallic materials

Metal honeycombs involve operations that are best classified as s/m forming processes

Two basic methods of manufacturing honeycomb materials:-Expansion process is the more common of the two

-The stacked sheets with bands of adhesive are heated (cured) to create strong bonds at the adhesive joints; then sliced & pulled apart

-In corrugation process: adhesive is applied at node lines, corrugated sheets are stacked in block, then curedstacked in block, then cured

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Material scrap is the primary concern in blanking operationsNote careful design of the part (with full knowledge of process of manufacture) to minimize scrap in these three examples

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In bending operations, the main concerns are:-Material fracture-Wrinkling-Inability to form the bend

A s/m part with a flange that is to be bent will force the flange to undergo compression-The compression can cause buckling

Note improvement by cutting a relief notch (middle), or by redesign of part (at right)

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Right angle bends are susceptible to tearing and wrinkling

Three illustrations of utilizing a notch at the bend to minimize tearing

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The bend radius is a highly stressed area.Therefore, desirable to remove all stress concentrations from this area

Holes near bends: undesirable-First: can holes/bends be redesigned to be further apart?-If not, then may use a crescent or an ear as shown in these illustrations (a)

Bending flanges-Avoid tabs & notches-When tabs are necessary, use large radii to reduce stress concentrations

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Bending & notches- Important to orient notches properly wrt grain direction

Fig. 16.17: Bends s/b perpendicular to rolling direction (or at least at an angle to it) to avoid cracking

Fig. 16.59: Bending sharp radii can be accomplished through scoring or embossing; but this can result in fracturing

Ensure burrs are not present in a bend allowance (since they are brittle because of strain hardening) and can fracture

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Sheet Metal Forming Equipment- The basic equipment consists of mechanical, hydraulic, pneumatic or hydraulic-pneumatic presses with variety of features & computer controls

-Traditional C frame is widely used for ease of tool & workpiece accessibility-C Frame is not as stiff as pillar or double column-Accessibility is a less important consideration for automation

-Variations on basic C Frame for wider pieces and ease of manual use & accessibility

Many s/m presses are very much smaller than the large machine illustrated hereMany s/m presses are very much smaller than the large machine illustrated here

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In generalOften a particular part or shape can be made in several alternate ways.

For example: A cup-shaped part can be formed by deep drawing, spinning, rubber forming, explosive forming, impact extrusion, casting, fabrication

Appropriate process is often a function of many variables, including volumes

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